This invention relates to a method of injecting a molten material such as molten metal or plasticizing resin into a mold under pressure, and also relates to an injection apparatus used in an injection molding system such as a die casting machine, a plastic molding machine and so forth, and capable of utilizing the method of this invention.
In a prior art die casting machine, the speed of injecting the molten material, i.e. the piston speed of the cylinder for injecting the molten material has been set as about 2 to 3 cm/sec thus far, and a typical program of injection has been designed as that shown in FIG. 1, where the ordinate represents the speed of the piston while the abscissa representing time. Explaining a typical example of the casting program in the prior art die casting machine in accordance with the figure above, it will be understood that the piston is initially moved at a slow speed from the beginning up to the time t.sub.1, and then it changes its motion to the high speed one when the time has become t.sub.1. At the same time, casting of molten material is commenced at this time t.sub.1, and then casting process is finished at the time t.sub.2. As seen in the casting program set forth above, the increase of the piston speed occurs very abruptly at the beginning of the high speed injection and the decrease equally the piston speed at the time when the filling of molten material has been completed. It has been generally said that the higher speed die casting can provide a product with better physical property. However, in the event that the casting process follows such a program set forth above that has a steep pattern at both rising up and falling down portion of piston speed, there have been problems set forth below.
1. From the beginning of filling up the mold with molten material to the end thereof, the piston is kept operated with high speed, where the piston pressure is about 140-240 kg/cm.sup.2, so that the kinetic energy of the piston is transmitted directly to the molten material, thus producing peak pressure in the molten material filling up the mold cavity, generating casting fins and, in the worst case, ejecting the molten material from the interface of two halves of the metal mold.
2. Even when the injection has been completed, the pressurized oil for pushing the piston has still a large kinetic energy. Thus, this kinetic energy is transmitted abruptly to the piston which is about to stop its motion completely, so that peak pressure in the molten material ought to be enhanced further, Then, as shown in FIG. 2, pressure vibration takes place in the pressurized oil. This vibration is transmitted to the molten material, which is about to solidify in the cavity, thereby the product cast being made porous or less densified in its inside. In FIG. 2, the ordinate represents the piston pressure while the abscissa representing time.